public interface Lock
Lock implementations provide more extensive locking
operations than can be obtained using synchronized methods
and statements. They allow more flexible structuring, may have
quite different properties, and may support multiple associated
Condition objects.
A lock is a tool for controlling access to a shared resource by
multiple threads. Commonly, a lock provides exclusive access to a
shared resource: only one thread at a time can acquire the lock and
all access to the shared resource requires that the lock be
acquired first. However, some locks may allow concurrent access to
a shared resource, such as the read lock of a ReadWriteLock.
The use of synchronized methods or statements provides
access to the implicit monitor lock associated with every object, but
forces all lock acquisition and release to occur in a block-structured way:
when multiple locks are acquired they must be released in the opposite
order, and all locks must be released in the same lexical scope in which
they were acquired.
While the scoping mechanism for synchronized methods
and statements makes it much easier to program with monitor locks,
and helps avoid many common programming errors involving locks,
there are occasions where you need to work with locks in a more
flexible way. For example, some algorithms for traversing
concurrently accessed data structures require the use of
"hand-over-hand" or "chain locking": you
acquire the lock of node A, then node B, then release A and acquire
C, then release B and acquire D and so on. Implementations of the
Lock interface enable the use of such techniques by
allowing a lock to be acquired and released in different scopes,
and allowing multiple locks to be acquired and released in any
order.
With this increased flexibility comes additional
responsibility. The absence of block-structured locking removes the
automatic release of locks that occurs with synchronized
methods and statements. In most cases, the following idiom
should be used:
Lock l = ...;
l.lock();
try {
// access the resource protected by this lock
} finally {
l.unlock();
}
When locking and unlocking occur in different scopes, care must be
taken to ensure that all code that is executed while the lock is
held is protected by try-finally or try-catch to ensure that the
lock is released when necessary.
Lock implementations provide additional functionality
over the use of synchronized methods and statements by
providing a non-blocking attempt to acquire a lock (tryLock()), an attempt to acquire the lock that can be
interrupted (lockInterruptibly(), and an attempt to acquire
the lock that can timeout (tryLock(long, TimeUnit)).
A Lock class can also provide behavior and semantics
that is quite different from that of the implicit monitor lock,
such as guaranteed ordering, non-reentrant usage, or deadlock
detection. If an implementation provides such specialized semantics
then the implementation must document those semantics.
Note that Lock instances are just normal objects and can
themselves be used as the target in a synchronized statement.
Acquiring the
monitor lock of a Lock instance has no specified relationship
with invoking any of the lock() methods of that instance.
It is recommended that to avoid confusion you never use Lock
instances in this way, except within their own implementation.
Except where noted, passing a null value for any
parameter will result in a NullPointerException being
thrown.
All Lock implementations must enforce the same
memory synchronization semantics as provided by the built-in monitor
lock, as described in
The Java Language Specification, Third Edition (17.4 Memory Model):
lock operation has the same memory
synchronization effects as a successful Lock action.
unlock operation has the same
memory synchronization effects as a successful Unlock action.
The three forms of lock acquisition (interruptible,
non-interruptible, and timed) may differ in their performance
characteristics, ordering guarantees, or other implementation
qualities. Further, the ability to interrupt the ongoing
acquisition of a lock may not be available in a given Lock
class. Consequently, an implementation is not required to define
exactly the same guarantees or semantics for all three forms of
lock acquisition, nor is it required to support interruption of an
ongoing lock acquisition. An implementation is required to clearly
document the semantics and guarantees provided by each of the
locking methods. It must also obey the interruption semantics as
defined in this interface, to the extent that interruption of lock
acquisition is supported: which is either totally, or only on
method entry.
As interruption generally implies cancellation, and checks for interruption are often infrequent, an implementation can favor responding to an interrupt over normal method return. This is true even if it can be shown that the interrupt occurred after another action may have unblocked the thread. An implementation should document this behavior.
ReentrantLock,
Condition,
ReadWriteLock| Modifier and Type | Method and Description |
|---|---|
void |
lock()
Acquires the lock.
|
void |
lockInterruptibly()
Acquires the lock unless the current thread is
interrupted.
|
Condition |
newCondition()
Returns a new
Condition instance that is bound to this
Lock instance. |
boolean |
tryLock()
Acquires the lock only if it is free at the time of invocation.
|
boolean |
tryLock(long time,
TimeUnit unit)
Acquires the lock if it is free within the given waiting time and the
current thread has not been interrupted.
|
void |
unlock()
Releases the lock.
|
void lock()
If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired.
Implementation Considerations
A Lock implementation may be able to detect erroneous use
of the lock, such as an invocation that would cause deadlock, and
may throw an (unchecked) exception in such circumstances. The
circumstances and the exception type must be documented by that
Lock implementation.
void lockInterruptibly() throws InterruptedException
Acquires the lock if it is available and returns immediately.
If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
If the current thread:
InterruptedException is thrown and the current thread's
interrupted status is cleared.
Implementation Considerations
The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.
An implementation can favor responding to an interrupt over normal method return.
A Lock implementation may be able to detect
erroneous use of the lock, such as an invocation that would
cause deadlock, and may throw an (unchecked) exception in such
circumstances. The circumstances and the exception type must
be documented by that Lock implementation.
InterruptedException - if the current thread is
interrupted while acquiring the lock (and interruption
of lock acquisition is supported).boolean tryLock()
Acquires the lock if it is available and returns immediately
with the value true.
If the lock is not available then this method will return
immediately with the value false.
A typical usage idiom for this method would be:
Lock lock = ...;
if (lock.tryLock()) {
try {
// manipulate protected state
} finally {
lock.unlock();
}
} else {
// perform alternative actions
}
This usage ensures that the lock is unlocked if it was acquired, and
doesn't try to unlock if the lock was not acquired.true if the lock was acquired and
false otherwiseboolean tryLock(long time, TimeUnit unit) throws InterruptedException
If the lock is available this method returns immediately
with the value true.
If the lock is not available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until one of three things happens:
If the lock is acquired then the value true is returned.
If the current thread:
InterruptedException is thrown and the current thread's
interrupted status is cleared.
If the specified waiting time elapses then the value false
is returned.
If the time is
less than or equal to zero, the method will not wait at all.
Implementation Considerations
The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.
An implementation can favor responding to an interrupt over normal method return, or reporting a timeout.
A Lock implementation may be able to detect
erroneous use of the lock, such as an invocation that would cause
deadlock, and may throw an (unchecked) exception in such circumstances.
The circumstances and the exception type must be documented by that
Lock implementation.
time - the maximum time to wait for the lockunit - the time unit of the time argumenttrue if the lock was acquired and false
if the waiting time elapsed before the lock was acquiredInterruptedException - if the current thread is interrupted
while acquiring the lock (and interruption of lock
acquisition is supported)void unlock()
Implementation Considerations
A Lock implementation will usually impose
restrictions on which thread can release a lock (typically only the
holder of the lock can release it) and may throw
an (unchecked) exception if the restriction is violated.
Any restrictions and the exception
type must be documented by that Lock implementation.
Condition newCondition()
Condition instance that is bound to this
Lock instance.
Before waiting on the condition the lock must be held by the
current thread.
A call to Condition.await() will atomically release the lock
before waiting and re-acquire the lock before the wait returns.
Implementation Considerations
The exact operation of the Condition instance depends on
the Lock implementation and must be documented by that
implementation.
Condition instance for this Lock instanceUnsupportedOperationException - if this Lock
implementation does not support conditions